.\" Copyright (C) 2014 Kees Cook <keescook@chromium.org>
.\" and Copyright (C) 2012 Will Drewry <wad@chromium.org>
.\" and Copyright (C) 2008, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH SECCOMP 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
seccomp \- operate on Secure Computing state of the process
.SH SYNOPSIS
.nf
.B #include <linux/seccomp.h>
.B #include <linux/filter.h>
.B #include <linux/audit.h>
.B #include <linux/signal.h>
.B #include <sys/ptrace.h>
.\" Kees Cook noted: Anything that uses SECCOMP_RET_TRACE returns will
.\"                  need <sys/ptrace.h>

.BI "int seccomp(unsigned int " operation ", unsigned int " flags \
", void *" args );
.fi
.SH DESCRIPTION
The
.BR seccomp ()
system call operates on the Secure Computing (seccomp) state of the
calling process.

Currently, Linux supports the following
.IR operation
values:
.TP
.BR SECCOMP_SET_MODE_STRICT
The only system calls that the calling thread is permitted to make are
.BR read (2),
.BR write (2),
.BR _exit (2)
(but not
.BR exit_group (2)),
and
.BR sigreturn (2).
Other system calls result in the delivery of a
.BR SIGKILL
signal.
Strict secure computing mode is useful for number-crunching
applications that may need to execute untrusted byte code, perhaps
obtained by reading from a pipe or socket.

Note that although the calling thread can no longer call
.BR sigprocmask (2),
it can use
.BR sigreturn (2)
to block all signals apart from
.BR SIGKILL
and
.BR SIGSTOP .
This means that
.BR alarm (2)
(for example) is not sufficient for restricting the process's execution time.
Instead, to reliably terminate the process,
.BR SIGKILL
must be used.
This can be done by using
.BR timer_create (2)
with
.BR SIGEV_SIGNAL
and
.IR sigev_signo
set to
.BR SIGKILL ,
or by using
.BR setrlimit (2)
to set the hard limit for
.BR RLIMIT_CPU .

This operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP
enabled.

The value of
.IR flags
must be 0, and
.IR args
must be NULL.

This operation is functionally identical to the call:

    prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
.TP
.BR SECCOMP_SET_MODE_FILTER
The system calls allowed are defined by a pointer to a Berkeley Packet
Filter (BPF) passed via
.IR args .
This argument is a pointer to a
.IR "struct\ sock_fprog" ;
it can be designed to filter arbitrary system calls and system call
arguments.
If the filter is invalid,
.BR seccomp ()
fails, returning
.BR EINVAL
in
.IR errno .

If
.BR fork (2)
or
.BR clone (2)
is allowed by the filter, any child processes will be constrained to
the same system call filters as the parent.
If
.BR execve (2)
is allowed,
the existing filters will be preserved across a call to
.BR execve (2).

In order to use the
.BR SECCOMP_SET_MODE_FILTER
operation, either the caller must have the
.BR CAP_SYS_ADMIN
capability in its user namespace, or the thread must already have the
.I no_new_privs
bit set.
If that bit was not already set by an ancestor of this thread,
the thread must make the following call:

    prctl(PR_SET_NO_NEW_PRIVS, 1);

Otherwise, the
.BR SECCOMP_SET_MODE_FILTER
operation will fail and return
.BR EACCES
in
.IR errno .
This requirement ensures that an unprivileged process cannot apply
a malicious filter and then invoke a set-user-ID or
other privileged program using
.BR execve (2),
thus potentially compromising that program.
(Such a malicious filter might, for example, cause an attempt to use
.BR setuid (2)
to set the caller's user IDs to non-zero values to instead
return 0 without actually making the system call.
Thus, the program might be tricked into retaining superuser privileges
in circumstances where it is possible to influence it to do
dangerous things because it did not actually drop privileges.)

If
.BR prctl (2)
or
.BR seccomp ()
is allowed by the attached filter, further filters may be added.
This will increase evaluation time, but allows for further reduction of
the attack surface during execution of a thread.

The
.BR SECCOMP_SET_MODE_FILTER
operation is available only if the kernel is configured with
.BR CONFIG_SECCOMP_FILTER
enabled.

When
.IR flags
is 0, this operation is functionally identical to the call:

    prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

The recognized
.IR flags
are:
.RS
.TP
.BR SECCOMP_FILTER_FLAG_TSYNC
When adding a new filter, synchronize all other threads of the calling
process to the same seccomp filter tree.
A "filter tree" is the ordered list of filters attached to a thread.
(Attaching identical filters in separate
.BR seccomp ()
calls results in different filters from this perspective.)

If any thread cannot synchronize to the same filter tree,
the call will not attach the new seccomp filter,
and will fail, returning the first thread ID found that cannot synchronize.
Synchronization will fail if another thread in the same process is in
.BR SECCOMP_MODE_STRICT
or if it has attached new seccomp filters to itself,
diverging from the calling thread's filter tree.
.RE
.SS Filters
When adding filters via
.BR SECCOMP_SET_MODE_FILTER ,
.IR args
points to a filter program:

.in +4n
.nf
struct sock_fprog {
    unsigned short      len;    /* Number of BPF instructions */
    struct sock_filter *filter; /* Pointer to array of
                                   BPF instructions */
};
.fi
.in

Each program must contain one or more BPF instructions:

.in +4n
.nf
struct sock_filter {            /* Filter block */
    __u16 code;                 /* Actual filter code */
    __u8  jt;                   /* Jump true */
    __u8  jf;                   /* Jump false */
    __u32 k;                    /* Generic multiuse field */
};
.fi
.in

When executing the instructions, the BPF program operates on the
system call information made available (i.e., use the
.BR BPF_ABS
addressing mode) as a (read-only)
.\" Quoting Kees Cook:
.\"     If BPF even allows changing the data, it's not copied back to
.\"     the syscall when it runs. Anything wanting to do things like
.\"     that would need to use ptrace to catch the call an directly
.\"     modify the registers before continuing with the call.
buffer of the following form:

.in +4n
.nf
struct seccomp_data {
    int   nr;                   /* System call number */
    __u32 arch;                 /* AUDIT_ARCH_* value
                                   (see <linux/audit.h>) */
    __u64 instruction_pointer;  /* CPU instruction pointer */
    __u64 args[6];              /* Up to 6 system call arguments */
};
.fi
.in

Because numbering of system calls varies between architectures and
some architectures (e.g., x86-64) allow user-space code to use
the calling conventions of multiple architectures, it is usually
necessary to verify the value of the
.IR arch
field.

It is strongly recommended to use a whitelisting approach whenever
possible because such an approach is more robust and simple.
A blacklist will have to be updated whenever a potentially
dangerous system call is added (or a dangerous flag or option if those
are blacklisted), and it is often possible to alter the
representation of a value without altering its meaning, leading to
a blacklist bypass.

The
.IR arch
field is not unique for all calling conventions.
The x86-64 ABI and the x32 ABI both use
.BR AUDIT_ARCH_X86_64
as
.IR arch ,
and they run on the same processors.
Instead, the mask
.BR __X32_SYSCALL_BIT
is used on the system call number to tell the two ABIs apart.
.\" As noted by Dave Drysdale in a note at the end of
.\" https://lwn.net/Articles/604515/
.\"     One additional detail to point out for the x32 ABI case:
.\"     the syscall number gets a high bit set (__X32_SYSCALL_BIT),
.\"     to mark it as an x32 call.
.\"
.\"     If x32 support is included in the kernel, then __SYSCALL_MASK
.\"     will have a value that is not all-ones, and this will trigger
.\"     an extra instruction in system_call to mask off the extra bit,
.\"     so that the syscall table indexing still works.

This means that in order to create a seccomp-based
blacklist for system calls performed through the x86-64 ABI,
it is necessary to not only check that
.IR arch
equals
.BR AUDIT_ARCH_X86_64 ,
but also to explicitly reject all system calls that contain
.BR __X32_SYSCALL_BIT
in
.IR nr .

The
.I instruction_pointer
field provides the address of the machine-language instruction that
performed the system call.
This might be useful in conjunction with the use of
.I /proc/[pid]/maps
to perform checks based on which region (mapping) of the program
made the system call.
(Probably, it is wise to lock down the
.BR mmap (2)
and
.BR mprotect (2)
system calls to prevent the program from subverting such checks.)

When checking values from
.IR args
against a blacklist, keep in mind that arguments are often
silently truncated before being processed, but after the seccomp check.
For example, this happens if the i386 ABI is used on an
x86-64 kernel: although the kernel will normally not look beyond
the 32 lowest bits of the arguments, the values of the full
64-bit registers will be present in the seccomp data.
A less surprising example is that if the x86-64 ABI is used to perform
a system call that takes an argument of type
.IR int ,
the more-significant half of the argument register is ignored by
the system call, but visible in the seccomp data.

A seccomp filter returns a 32-bit value consisting of two parts:
the most significant 16 bits
(corresponding to the mask defined by the constant
.BR SECCOMP_RET_ACTION )
contain one of the "action" values listed below;
the least significant 16-bits (defined by the constant
.BR SECCOMP_RET_DATA )
are "data" to be associated with this return value.

If multiple filters exist, they are \fIall\fP executed,
in reverse order of their addition to the filter tree\(emthat is,
the most recently installed filter is executed first.
(Note that all filters will be called
even if one of the earlier filters returns
.BR SECCOMP_RET_KILL .
This is done to simplify the kernel code and to provide a
tiny speed-up in the execution of sets of filters by
avoiding a check for this uncommon case.)
.\" From an Aug 2015  conversation with Kees Cook where I asked why *all*
.\" filters even if one of the early filters returns SECCOMP_RET_KILL:
.\"
.\"     It's just because it would be an optimization that would only speed up
.\"     the RET_KILL case, but it's the uncommon one and the one that doesn't
.\"     benefit meaningfully from such a change (you need to kill the process
.\"     really quickly?). We would speed up killing a program at the (albeit
.\"     tiny) expense to all other filtered programs. Best to keep the filter
.\"     execution logic clear, simple, and as fast as possible for all
.\"     filters.
The return value for the evaluation of a given system call is the first-seen
.BR SECCOMP_RET_ACTION
value of highest precedence (along with its accompanying data)
returned by execution of all of the filters.

In decreasing order of precedence,
the values that may be returned by a seccomp filter are:
.TP
.BR SECCOMP_RET_KILL
This value results in the process exiting immediately
without executing the system call.
The process terminates as though killed by a
.B SIGSYS
signal
.RI ( not
.BR SIGKILL ).
.TP
.BR SECCOMP_RET_TRAP
This value results in the kernel sending a
.BR SIGSYS
signal to the triggering process without executing the system call.
Various fields will be set in the
.I siginfo_t
structure (see
.BR sigaction (2))
associated with signal:
.RS
.IP * 3
.I si_signo
will contain
.BR SIGSYS .
.IP *
.IR si_call_addr
will show the address of the system call instruction.
.IP *
.IR si_syscall
and
.IR si_arch
will indicate which system call was attempted.
.IP *
.I si_code
will contain
.BR SYS_SECCOMP .
.IP *
.I si_errno
will contain the
.BR SECCOMP_RET_DATA
portion of the filter return value.
.RE
.IP
The program counter will be as though the system call happened
(i.e., it will not point to the system call instruction).
The return value register will contain an architecture\-dependent value;
if resuming execution, set it to something appropriate for the system call.
(The architecture dependency is because replacing it with
.BR ENOSYS
could overwrite some useful information.)
.TP
.BR SECCOMP_RET_ERRNO
This value results in the
.B SECCOMP_RET_DATA
portion of the filter's return value being passed to user space as the
.IR errno
value without executing the system call.
.TP
.BR SECCOMP_RET_TRACE
When returned, this value will cause the kernel to attempt to notify a
.BR ptrace (2)-based
tracer prior to executing the system call.
If there is no tracer present,
the system call is not executed and returns a failure status with
.I errno
set to
.BR ENOSYS .

A tracer will be notified if it requests
.BR PTRACE_O_TRACESECCOMP
using
.IR ptrace(PTRACE_SETOPTIONS) .
The tracer will be notified of a
.BR PTRACE_EVENT_SECCOMP
and the
.BR SECCOMP_RET_DATA
portion of the filter's return value will be available to the tracer via
.BR PTRACE_GETEVENTMSG .

The tracer can skip the system call by changing the system call number
to \-1.
Alternatively, the tracer can change the system call
requested by changing the system call to a valid system call number.
If the tracer asks to skip the system call, then the system call will
appear to return the value that the tracer puts in the return value register.

.\" This was changed in ce6526e8afa4.
.\" A related hole, using PTRACE_SYSCALL instead of SECCOMP_RET_TRACE, was
.\" changed in arch-specific commits, e.g. 93e35efb8de4 for X86 and
.\" 0f3912fd934c for ARM.
Before kernel 4.8, the seccomp check will not be run again after the tracer is
notified.
(This means that, on older kernels, seccomp-based sandboxes
.B "must not"
allow use of
.BR ptrace (2)\(emeven
of other
sandboxed processes\(emwithout extreme care;
ptracers can use this mechanism to escape from the seccomp sandbox.)
.TP
.BR SECCOMP_RET_ALLOW
This value results in the system call being executed.
.SH RETURN VALUE
On success,
.BR seccomp ()
returns 0.
On error, if
.BR SECCOMP_FILTER_FLAG_TSYNC
was used,
the return value is the ID of the thread
that caused the synchronization failure.
(This ID is a kernel thread ID of the type returned by
.BR clone (2)
and
.BR gettid (2).)
On other errors, \-1 is returned, and
.IR errno
is set to indicate the cause of the error.
.SH ERRORS
.BR seccomp ()
can fail for the following reasons:
.TP
.BR EACCESS
The caller did not have the
.BR CAP_SYS_ADMIN
capability in its user namespace, or had not set
.IR no_new_privs
before using
.BR SECCOMP_SET_MODE_FILTER .
.TP
.BR EFAULT
.IR args
was not a valid address.
.TP
.BR EINVAL
.IR operation
is unknown; or
.IR flags
are invalid for the given
.IR operation .
.TP
.BR EINVAL
.I operation
included
.BR BPF_ABS ,
but the specified offset was not aligned to a 32-bit boundary or exceeded
.IR "sizeof(struct\ seccomp_data)" .
.TP
.BR EINVAL
.\" See kernel/seccomp.c::seccomp_may_assign_mode() in 3.18 sources
A secure computing mode has already been set, and
.I operation
differs from the existing setting.
.TP
.BR EINVAL
.\" See stub kernel/seccomp.c::seccomp_set_mode_filter() in 3.18 sources
.I operation
specified
.BR SECCOMP_SET_MODE_FILTER ,
but the kernel was not built with
.B CONFIG_SECCOMP_FILTER
enabled.
.TP
.BR EINVAL
.I operation
specified
.BR SECCOMP_SET_MODE_FILTER ,
but the filter program pointed to by
.I args
was not valid or the length of the filter program was zero or exceeded
.B BPF_MAXINSNS
(4096) instructions.
.TP
.BR ENOMEM
Out of memory.
.TP
.BR ENOMEM
.\" ENOMEM in kernel/seccomp.c::seccomp_attach_filter() in 3.18 sources
The total length of all filter programs attached
to the calling thread would exceed
.B MAX_INSNS_PER_PATH
(32768) instructions.
Note that for the purposes of calculating this limit,
each already existing filter program incurs an
overhead penalty of 4 instructions.
.TP
.BR ESRCH
Another thread caused a failure during thread sync, but its ID could not
be determined.
.SH VERSIONS
The
.BR seccomp ()
system call first appeared in Linux 3.17.
.\" FIXME . Add glibc version
.SH CONFORMING TO
The
.BR seccomp ()
system call is a nonstandard Linux extension.
.SH NOTES
Rather than hand-coding seccomp filters as shown in the example below,
you may prefer to employ the
.I libseccomp
library, which provides a front-end for generating seccomp filters.

The
.IR Seccomp
field of the
.IR /proc/[pid]/status
file provides a method of viewing the seccomp mode of a process; see
.BR proc (5).

.BR seccomp ()
provides a superset of the functionality provided by the
.BR prctl (2)
.BR PR_SET_SECCOMP
operation (which does not support
.IR flags ).

Since Linux 4.4, the
.BR prctl (2)
.B PTRACE_SECCOMP_GET_FILTER
operation can be used to dump a process's seccomp filters.
.\"
.SS Seccomp-specific BPF details
Note the following BPF details specific to seccomp filters:
.IP * 3
The
.B BPF_H
and
.B BPF_B
size modifiers are not supported: all operations must load and store
(4-byte) words
.RB ( BPF_W ).
.IP *
To access the contents of the
.I seccomp_data
buffer, use the
.B BPF_ABS
addressing mode modifier.
.IP *
The
.B BPF_LEN
addressing mode modifier yields an immediate mode operand
whose value is the size of the
.IR seccomp_data
buffer.
.SH EXAMPLE
The program below accepts four or more arguments.
The first three arguments are a system call number,
a numeric architecture identifier, and an error number.
The program uses these values to construct a BPF filter
that is used at run time to perform the following checks:
.IP [1] 4
If the program is not running on the specified architecture,
the BPF filter causes system calls to fail with the error
.BR ENOSYS .
.IP [2]
If the program attempts to execute the system call with the specified number,
the BPF filter causes the system call to fail, with
.I errno
being set to the specified error number.
.PP
The remaining command-line arguments specify
the pathname and additional arguments of a program
that the example program should attempt to execute using
.BR execv (3)
(a library function that employs the
.BR execve (2)
system call).
Some example runs of the program are shown below.

First, we display the architecture that we are running on (x86-64)
and then construct a shell function that looks up system call
numbers on this architecture:

.nf
.in +4n
$ \fBuname -m\fP
x86_64
$ \fBsyscall_nr() {
    cat /usr/src/linux/arch/x86/syscalls/syscall_64.tbl | \\
    awk '$2 != "x32" && $3 == "'$1'" { print $1 }'
}\fP
.in
.fi

When the BPF filter rejects a system call (case [2] above),
it causes the system call to fail with the error number
specified on the command line.
In the experiments shown here, we'll use error number 99:

.nf
.in +4n
$ \fBerrno 99\fP
EADDRNOTAVAIL 99 Cannot assign requested address
.in
.fi

In the following example, we attempt to run the command
.BR whoami (1),
but the BPF filter rejects the
.BR execve (2)
system call, so that the command is not even executed:

.nf
.in +4n
$ \fBsyscall_nr execve\fP
59
$ \fB./a.out\fP
Usage: ./a.out <syscall_nr> <arch> <errno> <prog> [<args>]
Hint for <arch>: AUDIT_ARCH_I386: 0x40000003
                 AUDIT_ARCH_X86_64: 0xC000003E
$ \fB./a.out 59 0xC000003E 99 /bin/whoami\fP
execv: Cannot assign requested address
.in
.fi

In the next example, the BPF filter rejects the
.BR write (2)
system call, so that, although it is successfully started, the
.BR whoami (1)
command is not able to write output:

.nf
.in +4n
$ \fBsyscall_nr write\fP
1
$ \fB./a.out 1 0xC000003E 99 /bin/whoami\fP
.in
.fi

In the final example,
the BPF filter rejects a system call that is not used by the
.BR whoami (1)
command, so it is able to successfully execute and produce output:

.nf
.in +4n
$ \fBsyscall_nr preadv\fP
295
$ \fB./a.out 295 0xC000003E 99 /bin/whoami\fP
cecilia
.in
.fi
.SS Program source
.nf
#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>

#define X32_SYSCALL_BIT 0x40000000

static int
install_filter(int syscall_nr, int t_arch, int f_errno)
{
    unsigned int upper_nr_limit = 0xffffffff;

    /* Assume that AUDIT_ARCH_X86_64 means the normal x86-64 ABI */
    if (t_arch == AUDIT_ARCH_X86_64)
        upper_nr_limit = X32_SYSCALL_BIT - 1;

    struct sock_filter filter[] = {
        /* [0] Load architecture from 'seccomp_data' buffer into
               accumulator */
        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 (offsetof(struct seccomp_data, arch))),

        /* [1] Jump forward 5 instructions if architecture does not
               match 't_arch' */
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 5),

        /* [2] Load system call number from 'seccomp_data' buffer into
               accumulator */
        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 (offsetof(struct seccomp_data, nr))),

        /* [3] Check ABI - only needed for x86-64 in blacklist use
               cases.  Use JGT instead of checking against the bit
               mask to avoid having to reload the syscall number. */
        BPF_JUMP(BPF_JMP | BPF_JGT | BPF_K, upper_nr_limit, 3, 0),

        /* [4] Jump forward 1 instruction if system call number
               does not match 'syscall_nr' */
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1),

        /* [5] Matching architecture and system call: don't execute
	       the system call, and return 'f_errno' in 'errno' */
        BPF_STMT(BPF_RET | BPF_K,
                 SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)),

        /* [6] Destination of system call number mismatch: allow other
               system calls */
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),

        /* [7] Destination of architecture mismatch: kill process */
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL),
    };

    struct sock_fprog prog = {
        .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
        .filter = filter,
    };

    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog)) {
        perror("seccomp");
        return 1;
    }

    return 0;
}

int
main(int argc, char **argv)
{
    if (argc < 5) {
        fprintf(stderr, "Usage: "
                "%s <syscall_nr> <arch> <errno> <prog> [<args>]\\n"
                "Hint for <arch>: AUDIT_ARCH_I386: 0x%X\\n"
                "                 AUDIT_ARCH_X86_64: 0x%X\\n"
                "\\n", argv[0], AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
        exit(EXIT_FAILURE);
    }

    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
        perror("prctl");
        exit(EXIT_FAILURE);
    }

    if (install_filter(strtol(argv[1], NULL, 0),
                       strtol(argv[2], NULL, 0),
                       strtol(argv[3], NULL, 0)))
        exit(EXIT_FAILURE);

    execv(argv[4], &argv[4]);
    perror("execv");
    exit(EXIT_FAILURE);
}
.fi
.SH SEE ALSO
.BR bpf (2),
.BR prctl (2),
.BR ptrace (2),
.BR sigaction (2),
.BR proc (5),
.BR signal (7),
.BR socket (7)
.sp
Various pages from the
.I libseccomp
library, including:
.BR scmp_sys_resolver (1),
.BR seccomp_init (3),
.BR seccomp_load (3),
.BR seccomp_rule_add (3),
and
.BR seccomp_export_bpf (3).
.sp
The kernel source files
.IR Documentation/networking/filter.txt
and
.IR Documentation/prctl/seccomp_filter.txt .
.sp
McCanne, S. and Jacobson, V. (1992)
.IR "The BSD Packet Filter: A New Architecture for User-level Packet Capture" ,
Proceedings of the USENIX Winter 1993 Conference
.UR http://www.tcpdump.org/papers/bpf-usenix93.pdf
.UE
